Conventionally, optical lenses such as glasses, contact lenses, camera lenses, and pickup lenses for optical discs including compact discs have been used in various fields. Under these circumstances, lenses having multiple foci (multi-focus lenses) that can accommodate for both far vision and near vision in glasses, contact lenses and the like for presbyopia, for example, are sometimes required.
As such a multi-focus lens, a diffraction lens such as that disclosed in Patent Literature 1, for example, is known to the public. The diffraction lens described in Patent Document 1 is provided with a diffraction grating having a relief on the lens surface, and is allowed to form two foci by the 0th order light and first order diffracted light taking advantage of the diffraction phenomenon of light that passes through the relief. In case of using such a diffraction lens as a bifocal lens for presbyopia, it is made possible to form two foci by means of assigning each focus of the 0th order light and first order diffracted light for far vision and near vision respectively.
However, in recent years, optical lenses with more number of foci are sometimes required. For instance, bifocal lenses assign the 0th order light and first order light for far vision and near vision respectively, as described above, resulting in a problem getting more recognized these days that allocating energy to the mid-section between the 0th order and first order light becomes more difficult and the contrast in the intermediate vision range gets too low.
Therefore, to be able to generate more number of foci, a diffraction lens that produces multiple foci by means of forming multiple areas with a different relief in each area in the lens radial direction is proposed in Patent Document 2, for example. However, the diffraction lens described in Patent Document 2 had a risk of failing to achieve the desired focal effect when the diameter of incident light beam varies by aperture or due to pupil shrinkage of human eyes. Especially in case of an ophthalmic lens, it is not necessarily possible to stably place a diffraction lens in the desired position relative to the pupil, even if it is designed with consideration of physiological pupil diameter of human eyes, which poses a risk of failing to achieve the desired focal effect.    Patent Document 1: U.S. Pat. No. 5,121,980    Patent Document 2: U.S. Pat. No. 7,188,949